In an engine according to the prior art, as customary, an ignition plug is disposed in the vicinity of the bore center of the cylinder, and intake and exhaust valves are arranged to straddle the ignition plug. The intake and exhaust valves have large diameters so as to enlarge the effective areas for the intake and exhaust. In case the ignition plug is disposed in the vicinity of the bore center of the cylinder, the valve or valves have its or their one-side inner wall or walls extending close to the inner circumference of the cylinder. Especially in the case of the intake valve, the flow of intake air is then blocked by the inner circumference of the cylinder so that a kind of masking effect results to increase the resistance to the intake air flow. It is, therefore, conceivable to provide a plurality of intake valves. However, if the ignition plug is left in the vicinity of the bore center of the cylinder, the aforementioned masking effect still remains adversely to affect the intake air flow. If the number of the intake valves is increased, on the other hand, it becomes difficult to make room for mounting the ignition plug.
A feature of the present invention has been conceived in view of the background thus far described with the objective of providing a four-cycle engine which has each cylinder equipped with at least three intake valves and in which the said masking effect is reduced so that the intake air may smoothly flow into the combustion chamber, while there is still adequate room for mounting an ignition plug.
In order to achieve the above objective, the present invention is constructed such that one intake valve is arranged in the vicinity of the bore center of a cylinder, and an ignition plug is arranged in the vicinity of an axis extending through the bore center of said cylinder, such that the remaining intake valves are arranged at one side of said axis whereas an exhaust valve is arranged at the other side of said axis, and all of said intake valves and said exhaust valves are inclined so that their respective valve stem ends are spaced from a plane which extends through said axis and in parallel with said cylinder.
Furthermore, in order to enlarge the effective area of the intake passage of an intake valve, it has earlier been conceived to provide a plurality of intake valves and a plurality of intake passages. Especially when three or more intake valves are provided, it is difficult in a practical cylinder head to provide independent communications between the plurality of intake valves and a like number of intake passages. It is, therefore, conceivable to supply intake air from two intake passages into the three or more intake valves. In this instance, generally speaking, the flow of the intake air is liable to become irregular. For example, in case one intake passage communicates with two of the three valves whereas the other intake passage communicates with the remaining valve, a difference is established in the flow rate of the intake air which fluctuates in accordance with changes in the r.p.m. of the engine, thereby remarkably to change the combustion state. This makes it difficult to ensure a smooth run from low to high speeds. On the other hand, in case, for example, two intake valves are disposed on a bore center axis extending through the bore center of the cylinder, whereas the remaining intake valves are arranged symmetrically with respect to that bore center axis, two intake passages fail to have an identical curvature if they are separately made to communicate with the two intake valves on the bore center axis. Due to this difference in the curvature, the flow rates of the intake air through the respective intake passages also become irregular to invite the disadvantage that the running operation fails to become smooth for changes in the engine r.p.m.
A feature of the present invention has been conceived in view of the background thus far described and as an objective an intake system for a four-cycle engine of the type in which each cylinder is equipped with at least three intake valves, and in which at least one of said intake valves is arranged in the vicinity of a bore center axis extending through the bore center of said cylinder whereas the remaining intake valves are arranged symmetrically with respect to said bore center axis, said intake system being enabled to ensure a smooth running operation from low to high speeds by smoothening the flow of intake air, thereby to make uniform the flow rate of the intake air to be supplied to the respective intake valves.
In order to achieve the above objective, the cylinder includes two intake passages that are formed generally symmetrically with respect to the above defined bore center axis and are made to communicate with both the intake valves, which are positioned at the same side thereof, and the intake valve is arranged in the vicinity of said bore center axis.
Also, it has been suggested to equip one cylinder with a plurality of valves for an intake system and/or an exhaust system thereby to reduce the intake and/or exhaust resistances. Then the plural valves are arranged in parallel with each other or one another and are simultaneously opened and closed by the use of a rocker arm or a valve lifter. In case a rocker arm is used, the weight of the moving parts such as the rocker arm is increased to invite the disadvantage that the valve actuating mechanism fails to be appropriate for a high speed running operaiton.
If a valve lifter is used, as previously suggested, cylindrical valve lifters are independently attached to the respective valves, and the cam shaft is formed with a plurality of cams which are independently held in sliding contact with respective valve lifters. As a result, the sum of the weights of the moving parts also increases until the valve actuating mechanism is not suitable for high speed running operation, and the formation of multiple cams on the cam shaft increases the number of machining steps. In this case, moreover, in order to retain a space for arranging the respective valve lifters, the gaps between the two adjacent valves cannot be narrowed and as a consequence there is another disadvantage, namely that the design of the valve arrangement is restricted.
In the known methods using valve lifters it is necessary to arrange the valves parallel to the corresponding cam shaft, and the valve stem ends of the respective valves are disposed one-dimensionally below the cam shaft. Therefore, in case the valve stem ends of the respective valves are arranged two-dimensionally with respect to the corresponding cam shaft, i.e., in case the valve stem ends are offset in a plurality of directions relative to the cam shaft, it has been impossible to use conventional valve lifters.
The feature of the present invention has been conceived in view of the background thus far described and has as an objective to provide a valve actuating mechanism for use with an overhead cam shaft type engine, in which the weight of moving parts is reduced to make a high speed running operation possible, to reduce the number of cams on a cam shaft, to freely set the gaps between any two adjacent valves, and to arrange the valve stem ends of the respective valves two-dimensionally with respect to the corresponding cam shaft.
In order to achieve this objective, there is provided a valve actuating mechanism for an overhead cam shaft type engine which has one cylinder equipped with a plurality of parallel valves for at least either of its intake and exhaust systems, said valve actuating mechanism being constructed to comprise: valve springs for biasing the plural valves in directions to be closed; valve lifters so held in a cylinder head that they can slide in the same directions of said valves; thrust plates sandwiched between said valve lifters and the respective valve stem ends of said plural valves and made movable together with said valve lifters; and cam shafts for thrusting said valve lifters toward said valves, whereby said plural valves are simultaneously opened and closed by said thrust plates.
Yet another feature of the present invention relates to a valve actuating mechanism for use with an overhead cam shaft type engine, which mechanism is equipped with a plurality of parallel valves for at least either of an intake system or an exhaust system.
Yet another optional feature of the invention is concerned with inclining three or more intake valves for each cylinder parallel to one another, and in an identical direction with respect to the cylinder and simultaneously opening and closing all of the intake valves by the use of a single valve lifter or a single thrust plate. In this instance, however, if the intake valves used have the same valve stem length so as to make the parts of the respective valves common, the combustion chamber becomes deeper especially in the vicinity of the bore center to reduce the compression ratio but to increase the S/V ratio (i.e., the ratio of the surface area to the volume of the combustion chamber) thereby to reduce the efficiency of the engine. Moreover, since a large step is formed in the inner wall of the combustion chamber, the combustion is liable to become irregular thereby to make it difficult to run the engine with a lean air-fuel mixture. This results in disadvantages that the fuel economy is deteriorated, and that the measures for the exhaust emission control are handicapped.
On the other hand, if the intake valve in the vicinity of the bore center is located deep in the combustion chamber, it becomes necessary to dispose the exhaust valve adjacent thereto at a shallow place in the combustion chamber to raise the compression ratio as much as possible. In this instance, the thickness at the cylinder head side, which is required to hold the valve seat of the exhaust valve is reduced and risks deformation of the valve seat by thermal strain. There arises yet another problem, that the durability in the vicinity of the valve seat is reduced.
The feature of the invention has been conceived in view of those disadvantages thus far described and has an objective to provide a cylinder head for use with a four-cycle internal combustion engine of the type in which at least three intake valves for each cylinder are disposed parallel to one another and at an inclination with respect to the cylinder, and in which one of the aforementioned intake valves is arranged in the vicinity of the bore center of the aforementioned cylinder, said cylinder head being enabled to increase the compression ratio, to reduce the S/V ratio, to improve the fuel economy and the exhaust emission and to prevent the thermal deformation of the valve seats while improving the durability of the same.
In order to achieve the above-specified objective, the engine is constructed such that according to this feature the valve seat of the intake valve in the vicinity of the bore center is more offset inwardly of a combustion chamber in parallel with the corresponding valve stem than the valve seats of the remaining intake valves, thereby to make the aforementioned combustion chamber shallower in the vicinity of the bore center.